In a display driver which drives a display panel, such as an OLED (organic light emitting diode) display panel and a liquid crystal display panel, voltage data corresponding to drive voltages to be supplied to the display panel may be generated from grayscale values of respective subpixels of respective pixels described in image data.
FIG. 1 is a graph illustrating one exemplary correspondence relationship between the grayscale value of a subpixel described in an image data and the value of a voltage data. In FIG. 1, the graph of the correspondence relationship between the grayscale value and the value of the voltage data is illustrated with an assumption that the voltage proportional to the value of the voltage data is programmed to each subpixel of each pixel of an OLED display panel, in relation to the processing of the image data in driving the OLED display panel. When the grayscale value of a certain subpixel is “0”, for example, the value of the voltage data associated with the subpixel of interest is set to “1023”; in this case, the subpixel of interest is programmed with a drive voltage corresponding to the value “1023” of the voltage data, that is, a drive voltage of 5V in the example illustrated in FIG. 1. It should be noted that the brightness is increased as the drive voltage is lowered when the OLED display panel is driven with voltage programming. It should be noted that the correspondence relationship between the grayscale value of a subpixel described in an image data and the value of the voltage data is also dependent on the type of display panel. For example, in driving a liquid crystal display panel, the correspondence relationship between the grayscale value of a subpixel and the value of a voltage data is determined in general so that the drive voltage is generated so as to increase the difference between the drive voltage and the voltage on the common electrode (that is, the common level) as the grayscale value of the subpixel is increased.
A correction may be performed on an image data to improve the image quality of the image displayed on a display panel. In a display device including an OLED display panel, for example, there exist variations in the properties of OLED light emitting elements included in respective subpixels (respective pixel circuits) and the variations in the properties may cause a deterioration of the image quality, including display mura. In such a case, the display mura can be suppressed by preparing correction data for respective subpixels of respective pixels of the OLED display panel and correcting the image data corresponding to the respective pixel circuits in response to the prepared correction data.
FIG. 2 illustrates one example of the circuit configuration in which corrected image data are generated by correcting input image data and voltage data are generated from the corrected image data. In the configuration illustrated in FIG. 2, a correction circuit 101 generates corrected image data by correcting input image data and a voltage data generator circuit 102 generates voltage data from the corrected image data. In FIG. 2, the circuit configuration is illustrated with an assumption that the input image data and the corrected image data both describe the grayscale value of each subpixel with eight bits.
One issue of the circuit configuration illustrated in FIG. 2 is that an appropriate correction cannot be achieved in some cases when the grayscale value of an input image data is close to the allowed maximum grayscale value or the allowed minimum grayscale value. FIG. 3 is an illustration illustrating this issue. With respect to a correction circuit 101 configured to perform a correction which increases the grayscale value as illustrated in FIG. 3, the grayscale value of the corrected image data may be saturated at the allowed maximum grayscale value, when an input image data having a grayscale value close to the allowed maximum grayscale value is supplied to the correction circuit 101. In this case, the value of the voltage data is also saturated and this may cause deterioration of the image quality. A similar problem may occur with respect to a correction circuit 101 configured to perform a correction which decreases the grayscale value, when an input image data having a grayscale value close to the allowed minimum grayscale value is supplied to the correction circuit 101.
This problem may be avoided by increasing the bit width of the corrected image data supplied to the voltage data generator circuit 102; however, the increase in the bit width of the corrected image data may increase the circuit size of the voltage data generator circuit 102.
Another issue of the circuit configuration illustrated in FIG. 2 is that direct correction of drive voltages supplied to the display panel cannot be achieved. Discussed below is the case when the voltage offset of a subpixel of a display panel is to be cancelled through correction in a display driver configured to generate drive voltages proportional to the values of voltage data. In this case, it is most preferable that the voltage data is corrected so as to cancel the voltage offset; however, the circuit configuration illustrated in FIG. 2 only allows indirectly correcting the value of the voltage data through correcting the input image data. The value of the voltage data obtained as a result of the correction on the image data is not equivalent to the value obtained by directly correcting the voltage data. This may cause a deterioration of the image quality.
As discussed above, there exists a technical need for suppressing the image quality deterioration when image data correction is performed in a display driver configured to generate voltage data corresponding to drive voltages to be supplied to a display panel from the grayscale values of respective subpixels of respective pixels described in image data.
It should be noted that Japanese Patent Application Publication No. 2005-17420 A discloses a technique related to a display device including an OLED display panel, in which correction data are stored for respective pixels in a memory, and drive voltages determined based on data obtained by adding the correction data stored in the memory to video signal data are applied to the drive transistors of the respective pixels.
Japanese Patent Application Nos. 2006-349966 A, 2007-279290 A, 2009-223070 A disclose display devices configured to perform gamma corrections on R, G and B signals, multiply multiplication correction values with multipliers and add offset correction values with adders.
Japanese Patent Application No. 2005-250121 A discloses a drive circuit for driving an electro-optical device, which stores in correction data storage means block correction data respectively associated with a plurality of blocks obtained by dividing a pixel array area and corrects the control data controlling the emitted light brightness on the basis of the block correction data.
Japanese Patent Application Publication No. 2010-237528 A discloses a technique for compensating brightness variations of light emitting elements by correcting the image signal in response to the time-dependent deterioration properties of the light emitting elements. In the technique disclosed in this publication, the value of estimated emitted light luminance of each light emitting element is calculated and a correction value is determined for each light emitting element to reduce the difference between the maximum and minimum values of the estimated emitted light luminance.